The present invention relates to a semiconductor memory device, and more particularly to an arrangement that can be effectively adapted to a semiconductor integrated circuit device provided with a read-only memory function which is capable of erasing data.
With an EPROM in which field-effect transistors having a floating gate serve as memory cells, it is one of the significant present technical goods to improve the efficiency for writing data and to reduce the time for writing data.
Analysis conducted by the inventors of the present invention reveals that the data are written into the memory cells in a manner as described below.
Namely, the voltage drops in a depletion region between a pinch-off point under the gate electrode and the drain region due to an electric current that flows between the source region and the drain region. Therefore, the electric field intensity becomes a maximum near the drain region giving rise to the generation of hot electrons. Most of the hot electrons flow as a drain current, but some of them are injected as data into the floating gate.
Generally, the writing efficiency varies in proportion to the difference .DELTA.V between a potential (electric potential) at a position where hot electrons are generated and a control gate voltage.
Therefore, it has been attempted to increase the writing efficiency by selecting the control gate voltage to be, for instance, about 21 volts and the drain voltage to be about 12 volts.
Through experiments and consideration of the results thereof, however, the inventors have found the fact that the writing efficiency could not be sufficiently improved. That is, generating hot electrons in a high-potential region close to the drain region make the voltage differenc .DELTA.V between the control gate and the region in which hot electrons are generated small.
The principle of EPROM operation has been disclosed, for example, in a journal "NIKKEI ELECTRONICS", Jan. 5, 1981.